This invention relates to a constant speed drive device to convert variable engine speed to constant speed for operation of synchronous electrical generators, using differential gears, particularly for use on aircraft.
The need for a highly efficient link, capable of bilateral power flow, connecting a variable speed shaft to a constant speed shaft is manyfold. A particular need is to drive an onboard aircraft alternator at constant speed while the turbine engine speed varies. Presently, two methods are employed to provide a constant frequency on aircraft:
1. Constant Speed Drive (CSD) PA0 2. Variable-Speed, Constant Frequency (VSCF) PA0 1. E. Ohno, T. Kishimoto, and M. Akamatsu, "The Thyristor Commutatorless Motor," IEEE Trans. Mag., Vol. MAG-3, September 1967, pp. 236-240. PA0 2. T. Tsachiya, "Basic Characteristics of Cycloconverter-Type Commutatorless Motors," IEEE Trans. IGA, Vol. IGA-7, No. 4, July-August 1970, pp. 349-356. PA0 3. N. Sato and V. V. Semenos, "Adjustable Speed Drive with a Brushless DC Motor," IEEE Trans. IGA, Vol. IGA-7, No. 4, July-August 1971, pp. 539-543. PA0 4. E. P. Cornell and D. W. Novotny, "Commutation by Armature Induced Voltage in Self-Controlled Synchronous Machines," IEEE Trans. PAS, Vol. PAS-93, 1974, pp. 760-766. PA0 5. N. Sato, "A Brushless DC Motor with Armature Induced Voltage Commutation," IEEE Trans. PAS, Vol. PAS-91, July-August 1972, pp. 1485-1492. PA0 6. J. M. D. Murphy, Thyristor Control of AC Motors, (Pergamon Press, Oxford, 1973), pp. 140-149. PA0 7. F. J. Bourbeau, "Synchronous Motor Railcar Propulsion," IEEE Trans. IAS, Vol. IA-13 No. 1, January-February 1977, pp. 8-17. PA0 8. T. Maeno and M. Kobata, "AC Commutatorless and Brushless Motor," IEEE Trans. PAS, Vol. PAS-91, July-August 1972, pp. 1476-1484. PA0 9. Y. Shrinryo, I. Hosono, and K. Syoji, "Commutatorless DC Drive for Steel Rolling Mill," IEEE-IGA Conference Record, 1977Annual Meeting, pp. 263-271. PA0 10. A. C. Williamson, N. A. H. Issa, and A. R. A. M. Makky, "Variable-Speed Inverter-Fed Synchronous Motor Employing Natural Commutation," Proc. IEEE, Vol. 125, No. 2, Feb. 1978, pp. 118-120. PA0 11. N. A. Demardash, T. W. Nehl, and E. Maslowski, "Dynamic Modeling of Brushless DC Motors in Electric Propulsion and Electromechanical Actuation by Digital Techniques," IEEE IAS Conference Record, 1980 Annual Meeting, September 28-October 3 1980, pp. 570-579.
The VSCF system allows the alternator shaft to vary directly with turbine speed. The variable frequency alternator output is then conditioned by a cycloconverter to obtain a constant frequency. The VSCF system is not sensitive to attitude changes, and thus, functions well on highly maneuverable aircraft. However, total output power of the alternator must pass through the cycloconverter, leading to bulky and expensive power conditioning and filter circuitry.
The CSD scheme utilizes a mechanical differential which mechanically sums the input of two shafts and outputs this sum to a third shaft. Connected on this third shaft is a constant speed synchronous generator. Connected to one of the input shafts is the turbine (nost likely through gearing). The second input shaft is connected to a speed compensating device which accounts for engine speed changes. The speed conpensating device has been a hydraulic motor supplied by a hydraulic pumping mechanism driven from the engine. A constant alternator shaft speed is maintained by proper clockwise or counterclockwise rotation of the differential carrier housing through use of a reversible hydraulic pump-motor drive. For a 1.7:1 turbine speed range and a lossless system, a maximum of 21.5% of the alternator shaft power must pass through the compensating hydraulic drive, while 78.5% to 100% of the power is transmitted directly through the differential gearing. The hydraulic CSD's are extremely sensitive to attitude and require special oil systems and filling procedures to ensure proper operation during all flight modes. Without the special oil systems, there is a problem during maneuvers that produce negative gravity. In such cases, fluid level shifts can cause the hydraulic system to momentarily malfunction, creating an out-of-frequency range condition and leading to loss of electrical power.
Regardless of the above described potential failure mode, the concept of the CSD system has a quite desirable feature in that a large percentage of its output power is transmitted only through a low-order-mesh gear train, which by nature is highly efficient.
The following items relating to electrical machinery are referenced in the detailed description: